The present invention relates to a method and apparatus for diagnosing damage to electrically conductive materials, e.g. corrosion or fatigue, by using an ultrasonic resonance method. With the ultrasonic resonance method, ultrasonic waves are transmitted and received through a conductive material by an EMAT (Electro-Magnetic Acoustic Transducer) disposed adjacent to the surface of the conductive material. More specifically, the present invention relates to a method and apparatus for diagnosing by using an EMAT a damaged condition, e.g. corrosion, creep, stress-corrosion crack, or fatigue, of a piled up or laminated structure of conductive materials fastened together with rivets, bolts or the like.
Electro-magnetic acoustic transducers (EMATs) have been disclosed, for example, in the Gazette of Japanese Patent No. 3052049 and the Gazette of Japanese Patent No. 3052050. FIG. 1 is a diagram for explaining the principle of a publicly known electro-magnetic acoustic transducer (EMAT). As shown in FIG. 1, the EMAT 20 includes a pair of permanent magnets 3a and 3b disposed adjacent to the upper surface of a conductive material (object to be measured) 2. The EMAT 20 further includes a driver coil 5 and a detection coil 15. The pair of permanent magnets 3a and 3b are placed adjacent to each other with a spacer 3c interposed therebetween. The permanent magnets 3a and 3b generate magnetic fields opposite in direction to each other with respect to the conductive material 2. The permanent magnets 3a and 3b may be electromagnets arranged to generate similar magnetic fields. The driver coil 5 and the detection coil 15 are flat plate-shaped coils, which are disposed between the permanent magnets 3a and 3b on the one hand and the upper surface of the conductive material 2 on the other. The conductive material (object to be measured) 2 may be covered with a non-conductive material because the EMAT can transmit and receive ultrasonic waves in a non-contact manner.
The permanent magnets 3a and 3b form a static magnetic field 4 in the thickness direction of the conductive material 2. When the driver coil 5 is supplied with a radio-frequency burst current 7 from a controller 18, an eddy current 8 is generated on the surface of the conductive material 2 in the direction reverse to that of the current 7. The eddy current 8 and the static magnetic field 4 interact each other to generate a Lorentz force 9 according to the Fleming's left-hand rule. The Lorentz force 9 acts on free electrons in the conductive material 2, causing the free electrons to collide with ions and so forth, thereby inducing motions in a direction perpendicular to the directions of the static magnetic field 4 and the radio-frequency current 7 inside the conductive material 2, and thus generating ultrasonic shear waves 10.
The ultrasonic shear waves 10 travel in the direction of the arrow 11 in FIG. 1. The ultrasonic shear waves 10 are reflected by the upper or lower surface of the conductive material 2 and also reflected by flaws, defects, grain boundaries, changes of material's structure, etc. in the conductive material 2 to travel in the direction of the arrow 12. When the reflected ultrasonic shear waves 10 reach the vicinity of the upper surface of the conductive material 2, a force 13 is generated. The force 13 and the static magnetic field 4 interact each other to generate an eddy current 14. The eddy current 14 is detected with the detection coil 15. The detected current is amplified by a pre-amplifier 16 and a main amplifier 17 and sent to the controller 18. The controller 18 analyzes the current received from the main amplifier 17 to evaluate flaws, defects, grain boundaries, changes of material's structure, etc. in the conductive material 2.
The gazette of Japanese Patent No. 3052049 discloses an electro-magnetic acoustic transducer in which a transmission coil for generating ultrasonic waves and a reception coil for detecting ultrasonic waves are each formed into a spiral elongated shape and disposed to overlie one another with an insulating sheet interposed therebetween, and a grounded portion common to the two coils is formed to extend through a through-hole in the insulating sheet, thereby making the coil unit compact in size.
The gazette of Japanese Patent No. 3052050 discloses an electro-magnetic acoustic transducer in which a transmission coil for generating ultrasonic waves and a reception coil for detecting ultrasonic waves are formed into a meandering shape and disposed to overlie one another with an insulating sheet interposed therebetween, and grounded portions of the coils are connected in common through a through-hole in the insulating sheet, thereby making the coil unit compact in size and allowing diagnosis of flaws, defects, grain boundaries, changes of material's structure, etc. in a cylindrical conductive material 2.